Women who suffer from depression or anxiety may be more likely to have underweight babies -- even when those babies are born at full term, after a normal-length pregnancy -- a new study shows. This study adds to a body of research that gives conflicting evidence about the role of mental health in pregnancy.

The study assessed 583 women in rural Bangladesh. (Similar studies have been carried out in the U.S. and elsewhere in the developed world. But since the contribution of social factors to depression and anxiety is unknown, it has been useful to analyze the relationship of depression and pregnancy health across many different social settings.) Of all the Bangladeshi women who participated in this study, 18% showed depressive symptoms, and 26% showed symptoms of general anxiety -- all as measured by a standardized international diagnostic test, given during the third trimester of pregnancy.

There was no real difference between the women with symptoms and the women without symptoms in terms of how many babies were born prematurely. But full-term babies born to women with depression or anxiety were about twice as likely to be born at low birth weight -- weighing less than 2,500 g (or 5.5 lbs) at delivery. They also had, on average, smaller head circumference. In Bangladesh, poverty and maternal malnutrition are big predictors of low birth weight in newborns, too. But the effects of depression and anxiety in this study seemed to hold among mothers of all incomes and all nutritional statuses.

The finding matters because small newborns are at much higher risk of health problems and infant death than babies born just a little bit bigger. Unfortunately, it's not clear at all why some studies -- like this new one today, published in the journal BMC Public Health -- find such a strong association between depression / anxiety and fetal development, while others do not. Worldwide, the evidence is decidedly mixed.Perhaps the conflicting results are something to do with the way that depression and anxiety are measured from one study to the next. Or perhaps mental-health problems are merely signals in some populations for other unknown and unmeasured health problems that can cause poor fetal development. Remarkably little is known, in fact, about the fetal-development processes that lead to premature delivery or low birth weight. In the U.S., however, these poorly understood pregnancy outcomes drive huge racial disparities in babies' health, with black infants dying at roughly twice the rate of white infants.

Researchers suggest a handful of possible mechanisms by which mental health during pregnancy might directly affect babies' birth weight -- if indeed it does. Depressed women may be less likely to seek out adequate prenatal care or medical care while pregnant; they may not gain enough weight; or, perhaps, the neuroendocrine effects of depression and anxiety somehow throw off the normal fetal development process.